Electromagnetic wave guide structure



1960 G. T. KOHMAN ETAL 2,966,543

ELECTROMAGNETIC WAVE GUIDE STRUCTURE Filed Aug. 23, 1957 l// 1 Q A 5 5 9g Q k & P Q & Q

" l a N 5 a. r KOHZIEIAI INVENTORS' 15055 J. A. YOUNGJR.

A 'TTORNE V United States Patent ELECTROMAGNETIC WAVE GUIDE STRUCTUREGirard T. Kohman, Summit, Stewart E. Miller, Middletowu, Charles F. P.Rose, West Allenhurst, and James A. Young, Jr., Fair Haven, N .J.,assignors to Bell Telephone Laboratories, Incorporated, New York, N.Y.,a corporation of New York Filed Aug. 23, 1957, Ser. No. 679,835

4 Claims. (Cl. 333-95) This invention relates to electromagnetic wavetrans-.

mission systems and, more particularly, to an improved form oftransmission line for the circular electric or TE mode of wavepropagation.

In the copending applications of J. R. Pierce, Serial No. 416,315, filedMarch 15, 1954 (now Patent 2,848,695), and S. E. Miller, Serial No.416,316, filed March 15, 1954 (now Patent 2,848,696), and in the articleHelix Wave Guide by S. P. Morgan and J. A. Young, Jr., in the BellSystem Technical Journal, November 1956, pages 1347- 1384, it isdisclosed that a closely wound helical conductor of diameter greaterthan 1.2 free space wave-lengths is a transmission medium suitable forpropagating a properly excited circular electric TE mode. It is shownthat such a medium greatly minimizes the inherent tendency of this modeto degenerate into spurious modes, particularly the TM mode and to alesser extent the TE, and TE modes. In particular, it is shown that uponthis wave-guiding structure the TE mode may have a substantiallydifierent phase constant from the TM mode thereby providing decouplingbetween these modes. Furthermore, the helix is surrounded by a jacket ofelectrically dissipative material which introduces a large difference inthe attenuation constants presented to the spurious modes and the TEmode and that by virtue of this difference, degeneration is reducedwhether or not there is a difference in phase constant without asubstantial amount of energy being actually lost in the dissipativematerial. The dissipative jacket is in turn surrounded by a sheath togive mechanical strength and protection to the structure.

Such a transmission medium is ideally suited for long distancetransmission of wide band signals since the attenuation of the TE modedecreases with increasing frequency. Used in long lengths, the helicalwave guide serves to negotiate both accidentally and intentionallyintroduced bends and turns. Used in shorter lengths, the helical guideserves as a filter to purify the TE energy and to remove spuriouscomponents, particularly of the TB and TE modes.

It is, therefore, an object of the present invention to improve thehelical wave guide transmission media from both an electrical and amechanical standpoint.

Extensive research and analysis have indicated that improved and optimumoperation of a helical transmission line can be obtained by providingthe lossy jacket with a resistivity in the range between 1 and ohmcentimeters, a range substantially below that heretofore contemplated.In the structures disclosed in the above-mentioned applications andpublication, the lossy jacket comprises a suitable plastic or dielectricmaterial in which small particles of resistive material are suspended.When the resistive material is carbon or similar material, an attempt tomore densely impregnate the plastic material to increase theconcentration of the resistive particles and lower the resistance isunsatisfactory because it is accompanied by a physical weakening of thematerial substantially before a resistivity of 10 ohm centimeters can bereached. When 2,966,643 Patented Dec. 27, 1960 a more highly conductivematerial such as powdered iron is used, increased concentration, whichbrings the resistive particles closer together, increases thecapacitance therebetween and tends to increase the dielectric constantof the material to such an extent that radio frequency energy isprevented from penetrating into it for attenuation. At the other end ofthe resistance scale are continuous surfaces or conductive films, asopposed to isolated particles, of conductive metals and their moreresistive oxides, but these all have r-esistivities substantially below1 ohm centimeter.

In accordance with the invention this apparent lack of a suitableresistance material is overcome by employing as the lossy jacket strandsof dielectric material that are coated with a thin metallicsemiconductive or resistive film and then laminated with a suitabledielectric plastic. While the real ohmic resistance of the film soemployed is much below the usable range, it will be shown that theefiective resistance presented by the laminated structure to thespurious mode energy can be readily controlled by the ratio of resistiveto dielectric material in the laminated structure. In particular,fibrous glass material is coated with a thin metallic oxide and thenlaminated with a synthetic resin such as epoxide. This structure hasdesirable electrical properties, is easy and economical to fabricate bycommercial processes. It has the further advantage in accordance withanother feature of the invention that additional layers of fibrous glassand plastic may be built as a homogeneous structure upon the lossyjacket layer to produce a protective sheath that has dimentionalaccuracy, that has moisture, corrosion, and deterioration resistance toa high degree, and that has a bending stilfness that can duplicate thatof a connected metallic-type wave guide. In addition, a dielectric layerbetween the helix and the resistive jacket may be added by thisconstruction for the purpose and in accordance with the teachings of H.G. Unger in his copending application Serial No. 679,929, filed August23, 1957.

These and other objects, the nature of the present invention, itsvarious features and advantages will appear more fully uponconsideration of the specific illustrative embodiment shown in theaccompanying drawings and analyzed in the following detailed descriptionthereof.

In the drawings:

Fig. 1 is a cut-away view of a small end portion of a transmission linein accordance with the invention; and

Fig. 2 is a cross-sectional view of the form upon which the structure ofFig. 1 may be made.

Referring specifically to Fig. 1, a cut-away cross-sectional view of asmall section of transmission line contemplated for use in a circularelectric mode wave transmission system is shown as an illustrativeembodiment of the present invention. This line comprises an elongatedconductive member 11 of relatively fine insulated wire closely wound ina helix. Conductor 11 may, for example, be a number 37 size, enameled orplastic insulated solid copper wire (0.005 inch overall diameter). Auniform and appropriate spacing between successive turns is provided bythe insulation.

Helix 11 is surrounded by successive layers or jackets which will eachbe described in more detail hereinafter. Immediately surrounding helixi1 is a dielectric jacket 12 of glass fibers impregnated with a plasticor resinous material. This is followed by a lossy jacket 13 of glassfibers covered with a film of metallic oxide and similarly impregnatedwith plastic. Jacket 13 is in turn surrounded by an outer jacket 14 ofimpregnated glass fibers. The right-hand portion of jacket 14 includes aconnector comprising an internally threaded region 15 for connecting toan adjoining section of wave guide, an internally smooth portion 16 oflarger diameter which facilitates thread alignment when inserting theadjoining section, and a larger diameter section 18 forming a seat 17 toreceive a suitable washer for sealing with the adjoining section. Acopper ring 19 between helix 11 and threaded portion 15 that has aninside diameter equal to the inside diameter of helix 11 terminates theends of the helix and forms a conductive abutting surface for theadjoining wave guide.

Fig. 2 shows the mandrel upon which the structure of Fig. l is formed.Portion 21 represents a smoothly polished member upon which helix 11 iswound and portion 22 represents a typical one of a pair of end moldssuitably fastened by threads 23 to either end of portion 21. Portion 22includes an externally screw threaded portion 24, a smooth portion 25and a seat forming portion 26. Such a mandrel of the desired lengthtogether with its end molds is mounted for axial rotation between thechucks of a suitable winding machine after rings 19 have been located inplace.

After a suitable mold release agent has been applied to mandrel 21 andend mold 22, the mandrel is rotated and helix 11 is closely Woundbetween rings 19. It may be wound with a single strand, or it may bewound with a plurality of strands being simultaneously fed in parallel.

Over this helix is laminated the first dielectric jacket 12,. thepurpose, function and dimensions of which are disclosed in more detailand claimed in the copending application of H. G. Unger, Serial No.679,929, filed August 23, 1957. In general, this jacket provides atransformation of the surface impedance that the lossy jacket 13presents to the longitudinal currents through helix 11 and acts as aninductance in parallel with the capacitance of the helix to compensatefor the shielding effect of the helix upon the spurious modes. Forfurther details, reference is made to said copending application.

The material of jacket 12 comprises plastic reinforced with glass fibersor glass fibers impregnated with plastic. Several methods of building upthis layer have each been found satisfactory. According to a first,cloth woven of glass fiber having a length similar to the length of thehelix is wound over helix 11 while the plastic is applied between eachlayer. Alternatively, a tape woven of glass fibers may be spirally wounddown and back a plurality of times along the helix. Similarly, glassfiber roving comprising a loosely twisted cord containing in the orderof 15 to 50 glass fibers is spirally wound down and back many timesuntil the required thickness is built up. In the case of roving, theloosely twisted fibers tend to flatten out so that each turn increasesthe layer by only the thickness of a few fibers. In the case of tape orroving, the material may first be passed through a container of fluidplastic before winding or the plastic may otherwise be suitably appliedbetween turns. The use of woven cloth appears preferable for handmade orcustom made short lengths while the use of tape or roving appearspreferable for use with commercial winding machines. In either case theglass content ofthe glass lamination should be held at a high uniformvalue perhaps of the order between 50 and 75 percent, the higher ratiosof glass appearing preferable.

Several plastic materials have been found suitable for practicing theinvention. A preferred embodiment employs a suitable commerciallyavailable epoxide resin of the type that may be catalytically cured toform a thermosetting polymer. For example, a suitable catalyst isusually either an amine, amide, or a combination of both. It ispreferred that a fast enough catalyst be employed that the exothermicheat aids in producing the cure. However, it has been found that thereaction may be hastened without undesirable shrinkage if the exothermiccure is also accompanied by moderate external heat. Specifically, 20parts per 100 by weight of the curing agent metaphenylene diamine hasproven satisfactory. Alternatively, one of the several less expensivethermoplastic polyester resins may be employed. In

-of light waves reflected from the thin oxide films.

general, in practicing this portion of the invention the techniques andmaterials used are similar to those known to the art and used in otherglass reinforced plastics. Reference is, therefore, made for furtherdetails and for alternative plastic materials to the text, GlassReinforced Plastics, by Phillip Morgan, published in the United Statesby Philosophical Library Incorporated.

Next, the resistive layer is laminated employing plastics of the typesdescribed and glass cloth, tape or roving having an electricallyconducting metallic oxide coating of the kind generally known as aniridized coating applied thereto. It is known that when glass or othervitreous ceramic bodies are heated and contacted with certain metallicsalts either in the form of fumes or atomized solutions thereof, astrongly adherent layer of an oxide of the metal is formed on itssurface. This process is known as iridizing because the coatings thusproduced are frequently iridescent due to the interference Oxidecoatings that have an electrical resistance and other characteristicssuitable for the present invention may be produced by using mixtures ofthe metal oxides tin, titanium, cadmium, indium and antimony. Inparticular, the combination of the oxides of tin plus small quantitiesof the oxides of titanium and antimony have been found satisfactory. Theselection and amount of the other materials combined with the tin oxidetogether with the thickness of the film control the electrical surfaceresistivity of the film. In general, the application of these materialsto glass from a tetrachloride solution is discussed in more detail inUnited States Patent 2,564,707, granted August 21, 1951, to J. M. Mocheland in the copending applications referred to therein.

The composition and thickness of the film thus formed, may be definedmore clearly after the function that the resistive jacket plays in thetransmission line has been examined; Thus, circular electric TE modeenergy is excited within helix 11 by its connection to a solid pipeguide or other component in the system in which the helix is to be usedeither as a filter or as a transmission medium for circular electricwave energy. A major component of the circular current of this wave isconducted along the helical path by each turn. Since the pitch of thehelix is small, this component constitutes substantially the entirecurrent of the wave. The wave is presented with only a small reactanceby the discontinuity between adjacent turns which changes the phasevelocity of the wave only slightly. Very little of the total TEo currentwill pass through the resistive material-of jacket 13 and, therefore,the attenuation constant of the TEm, wave is also substantiallyunchanged.

The TM mode has a predominantly longitudinal current flow along, thewave guiding path and will be seriously affected by the discontinuitybetween adjacent turns of helix 11. Not only is the phase constant ofthis mode increased by the reactance of each discontinuity, but thelongitudinal currents are forced to flow through the dissipativematerial of jacket 13'. Thus, the attenuation constant for the TM modebecomes very different from the attenuation constant for the TE mode andits degeneration into this spurious mode is reduced. With the TE and TEmodes, however, there is only a limited range of values of resistivityof jacket 13 for which these modes exhibit a preference for the jacketand are subject to its attenuation to a much greater extent than the TEmode. Calculations leading to this conclusion are fully set forth in thefirst above-identified publication. This range includes a resistancepresented to the wave energy in these modes of approximately 1 to 10 ohmcentimeters.

Now it is a particular feature of the present invention that as a resultof the laminated nature of jacket 13 the resistance presented'to thewave energy is not the same as the measurable resistance of the metallicfilms upon the glass. The latter will therefore be referred to as theohmic resistivity and the resistance presented to the wave as theeffective resistivity. By laminating the resistive films with dielectricmaterial in accordance with the invention, it is possible to use amaterial having an ohmic resistivity that is more than a thousand timeslower than the required effective resistivity. Thus, metallic andmetallic oxide films can be used with the further advantage that thedielectric constant of the composite material remains low as opposed tothe high dielectric constant of a composite material including isolatedmetallic particles.

To illustrate these principles, the parameters of a preferred embodimentmay be given by way of example. It is desired to produce an effectiveresistivity in the order of 2 ohm centimeters having a dielectricconstant in the order of 10. This is done by Winding 0.005 inch thicklaminations (the thickness of the glass fibers material) having ametallic oxide coating applied thereto of material having an ohmicresistivity of approximately 0.002 ohm centimeters to a total thicknessof 0.04 inch. The metallic oxide coating in such an embodiment may be inthe order of 3,000 angstroms in thickness which gives it a surfaceresistivity of approximately 50 ohms per square. Equivalent parametersfor other embodiments may be determined empirically or they may becalculated using the equations derived for multilayer transmission linestructures by E. I. Hawthorne in an article Electromagnetic ShieldingWith Transparent Coated Glass appearing in the Proceedings of theInstitute of Radio Engineers, vol. 42, page 548, March 1954.

Having thus constructed lossy jacket 13, the transmission line inaccordance with the invention is completed by adding the protectivesheath 14 and the connectors at each end thereof. Portions of theconnector comprising threaded portion 15, aligning portion 16 and seatportion 17-18 are formed by being filled in and built up to the outsidediameter of ring 19 in a wound or laminated construction by a processsimilar to that described for dielectric jacket 12. It is preferable toemploy roving of glass fibers and tape of glass fibers in these portionsand to employ a resin having some resiliency and ductility to preventchipping of these parts through use. Therefore, in accordance with apreferred embodiment of the invention, an epoxide resin cured by amixture of parts per 100 metaphenylene diamine and 32 parts per 100polyamide may be employed.

Finally, the structure of sheath 14 is wound over resistive jacket 13and over the formed threads and seats of parts 15, 16, 17 and 18. Exceptfor its greater thickness, sheath 14 may be identical to dielectricjacket 12. If preferred, somewhat larger fibers of glass may be usedhowever. Because of the identical mechanical nature of jackets 12, 13and 14, a substantially homogeneous, closely bounded structure isobtained. Thus, the thickness of sheath 14 is built up so that the totalthickness of the laminated structure has a bending stiffness comparableto that of the solid wall metallic wave guide employed in the samesystem: Such structural uniformity through the line is necessary toinsure uniform serpentine deformation of the entire line in the event ofthermal expansion. Otherwise, a concentrated bend would occur at theweakest point introducing undesirable mode conversion. In a particularembodiment, a wall /6 inch thick provides a sufiiciently largestructural moment of inertia to oifset the greater modulus of elasticityof the copper in the standard 2 inch inside diameter wave guide.

The completed guide is then cured according to standard plastic handlingprocesses while mandrel 21 continues to rotate. Slight elevations intemperature have been found permissible to hasten the curing. End molds22 may then be removed and mandrel 21 withdrawn.

In all cases, it is understood that the above-described arrangement isillustrative of one of the many possible specific embodiments thatrepresent applications of the principles of the invention. Numerous andvaried other arrangements can readily be devised in accordance withthese principles by those skilled in the art without departing from thespirit and scope of the invention.

What is claimed is:

1. A transmission medium for electromagnetic wave energy in the circularelectric mode, said medium comprising an elongated member of conductivematerial wound in a substantially helical form with adjacent turnselectrically insulated from each other, a first jacket surrounding saidhelix for presenting an effective resistivity of a given value to waveenergy having current components parallel to the axis of said helix,said jacket comprising glass fibers coated by an iridized film ofmetallic oxide, said film having a value of ohmic resistivitysubstantially lower than said given value, said coated fibers beingbound together in a laminated structure by a plastic material, a secondjacket surrounding said first jacket comprising further glass fibersbeing bound together in a laminated structure by a plastic material.

2. In combination With the medium of claim 1, a sec tion of conductivemetallic walled wave guide connected to one end of said medium, thelaminated jackets of said medium having a total thickness suflicentlygreater than the thckness or the metallic wall of said wave guide section so that said medium has a bending stiffness substantially equal tothe bending stiffness of said Wave guide section.

3. A transmission medium for electromagnetic wave energy in the circularelectric mode, said medium comprising an elongated member of conductivematerial wound in a substantially helical form with adjacent turnselectrically insulated from each other, and a jacket surrounding saidhelix for presenting an efiective resistivity of a given value to waveenergy having current compo nents parallel to the axis of said helix,said jacket comprising fibers of glass with iridized coatings ofmetallic oxide, said iridized coatings having values of ohmicresistivity substantially lower than said given value of efiectiveresistivity and being separated in a laminated construction bydielectric material at least a part of which c0mprises a plasticmaterial impregnating said glass fibers the amount of separation raisingthe value of effective resistivity presented to said component to saidgiven value.

4. The medium according to claim 3 wherein said fibers of glass arewoven into fabric and wherein said fabric is wound about the axis ofsaid helix.

References Cited in the file of this patent UNITED STATES PATENTS2,564,709 MOchel Aug. 21, 1951 2,745,074 Darling May 8, 1956 2,746,018Sichak May 15, 1956 2,779,006 Albersheim Jan. 22, 1957 FOREIGN PATENTS1,118,560 France Mar. 19, 1956 OTHER REFERENCES Publication, Fiberglass,published in catalog No. E1-44-7 by Owens-Corning Fiberglas Corp., page21.

